A sonnet takes different forms with the classical sonnet poets but there are two things that are common and they are the things that make a sonnet instantly recognisable.

A sonnet has fourteen lines

A sonnet is written in iambic pentameter

The fourteen lines required for a poem to be a sonnet are made up of rhyme patterns. There are different ways of organising the rhyme patterns. For example, the sonnet can be divided into two sections, each section having its own rhyme pattern. They are an eight line section, called an octet, and a six line section, called a sestet. That is the form used by the Italian poet, Petrarch, the most famous sonnet writer apart from Shakespeare. It’s known as the Petrarchan Sonnet or the Italian Sonnet.

The sonnet can also be divided into three four line sections, called quatrains, and a two line section called a couplet. This is the form Shakespeare uses and the form has become known as the Shakespearean Sonnet or the Elizabethan Sonnet.

The sonnet expresses a single idea but the division into octaves, sestets, quatrains and couplets allows the poet to switch the focus, dealing with a different aspect of the idea in each section..

The rhyme patters look like this. The octet is aabbaabba. All the ‘a’s rhyme with each other and all the ‘b’s rhyme with each other. The sestet is cdecde or cdcdcd or cddece. All the words ending the lines with the same letter rhyme with each other.

In Shakespeare’s sonnets, the quatrain patterns look like this. Abab cdcd efef and the couplet is gg. All the sonnets follow that pattern.

Iambic pentameter refers to the structure of the line. Iambic refers to the name of the foot, which is composed of a weaker syllable followed by an accented syllable. For example the word away has two syllables with a weak stress on the first, a, and a strong stress on the second, way. The word constitutes a foot or an iambus. Pentameter simply refers to the number of feet, in the case of the sonnet, five. So if you look at this line from a Shakespeare sonnet you will see how it works:

I never writ nor no man ever loved

There you have an iambic pentameter. All sonnets use that model and almost all the lines in Shakespeare’s plays are written in iambic pentameter as well.

So how do poets use this form? This is an example of a Petrarchan Sonnet written by William Wordsworth. Like other sonnet writers he uses iambic pentameter but, like Shakespeare, he sometimes disguises it to get the rhythm he wants to make it sound like everyday speech.

The Octave

Milton! thou shouldst be living at this hour: – A

England hath need of thee: she is a fen – B

Of stagnant waters: altar, sword, and pen, – B

Fireside, the heroic wealth of hall and bower, – A
Have forfeited their ancient English dower – A

Of inward happiness. We are selfish men; – B

Oh! raise us up, return to us again; – B

And give us manners, virtue, freedom, power. – A

The Sestet

Thy soul was like a Star, and dwelt apart; – C

Thou hadst a voice whose sound was like the sea: – D

Pure as the naked heavens, majestic, free, – D

So didst thou travel on life’s common way, – E

In cheerful godliness; and yet thy heart – C

The lowliest duties on herself did lay. – E

Wordsworth invokes the spirit of Milton who he thinks lived in a better time and the octet is concerned with all the faults he sees in the England of his own time. He switches the focus onto Milton himself in the sestet to show what greatness was in a past age.

This is Shakespeare writing about love:

First Quotrain

Let me not to the marriage of true minds -A

Admit impediments. Love is not love – B

Which alters when it alteration finds, – A

Or bends with the remover to remove: – B

Second Quotrain

Oh, no! it is an ever-fixéd mark, – C

That looks on tempests and is never shaken; D

It is the star to every wandering bark, -C

Whose worth’s unknown, although his height be taken. – D

Third Quotrain

Love’s not time’s fool, though rosy lips and cheeks – E

Within his bending sickle’s compass come’ – F

Love alters not with his brief hours and weeks, – E

But bears it out even to the edge of doom. – F

Couplet

If this be error and upon me proved, – G

I never writ, nor no man ever loved. – G

The first quatrain announces that love is perfect and can’t be affected by anything. The second quatrain elaborates on that, showing that love is something absolutely stable in a tumultuous world. The third quatrain switches the focus on to time and suggests that love lasts forever. The couplet is a final summing up in which the poet stakes his career on the truth of what he’s revealed about love.